Checkerboard
History
The Checkerboard cipher is a variation on a system devised by the Greek historian and writer, Polybius (circa. 200 BC - 120 BC) known as the Polybius square, a 25-letter alphabet written into a 5x5 grid with rows and columns labelled with a number.
Description
The Checkerboard cipher uses the same type of square as the Polybius but rows and columns are each labelled with either one or two 5- or 6-letter keywords. The cipher is created using a 5x5 square matrix filled with a keyed alphabet using all the letters of the alphabet, with J being replaced by I. If a 6 x 6 square matrix is selected the letters of the alphabet and numbers 0-9 are used with A followed by 1, B by 2, C by 3 and so on up to J followed by 0. A keyed alphabet is created from a key word, with repeated letters being omitted, followed by the unused letters of the alphabet in alphabetical order. For example using the key word BACKUP and appending unused alphabet letters produces the keyed alphabet:
BACKUPDEFGHILMNOQRSTVWXYZ
The keyed alphabet is written into the matrix in a selected route, e.g. horizontal, diagonal or clockwise. Using the row and column keys BRAIN and WAVES gives the following grid:
W A V E S
B B A C K U
R P D E F G
A H I L M N
I O Q R S T
N V W X Y Z
Encryption is performed by locating each plaintext letter in the keysquare and replacing it by the two letters indicating the row and column of its position. If two words are used to represent each row and column coordinate then either letter of the keywords can be used as the ciphertext.
Example
Encipher the following quote "I haven't lost my mind, it's backed up on disk somewhere.” using the keywords BACKUP BRAIN WAVES gives:
i h a v e n t l o s t m y m i n d i t s b a
AA AW BA NW RV AS IS AV IW IE IS AE NE AE AA AS RA AA IS IE BW BA
c k e d u p o n d i s k s o m e w h e r e
BV BE RV RA BS RW IW AS RA AA IE BE IE IW AE RV NA AW RV IV RV
Ciphertext:
AA AW BA NW RV AS IS AV IW IE IS AE NE AE AA AS RA AA IS IE BW BA BV BE RV RA BS RW IW AS RA AA IE BE IE IW AE RV NA AW RV IV RV
Solving
Solving method: Hill Climbing search.
On selecting the Solve button the dialog box below will appear prompting for the left and top keys to be entered. If more than 5 unique characters are found for either keyword a Double Checkerboard cipher type is assumed and two keywords will be required for each side. The letters in each side are shown in the dialog box.
If a Single Checkerboard is entered where there are exactly 5 unique letters found for each keyword, possible keywords will be selectable from a drop-down menu.
